The Three Secretaries 215 



Advancement of Science, was the result of three years' study. 

 This was based directly upon micrometrical measurements, 

 pictorial effect having been considered only so far as it was 

 incidental to minute fidelity. Even now, twenty-three years 

 after it was made, it is conceded that this drawing gives a 

 better idea of the minute structure of the surface of the sun 

 than is afforded by the best photographs. 



His paper on "The Minute Structure of the Solar Photo- 

 sphere," published in February, 1874, may be taken as a type 

 of his best work. 



" It possesses," writes Holden, " that hardly-definable qual- 

 ity by which we become aware that it was written from a full 

 mind. It is only fifteen pages long, yet we are not conscious 

 of undue brevity. One has a sense in reading that every 

 statement of fact, or every expression of opinion, is based 

 upon a hundred single instances like the one which is chosen, 

 or upon a hundred concurring judgments. It is not that you 

 are overborne by weight, but convinced by character. This 

 most important paper came at exactly the right time. It first 

 summarizes the works of other recent observers which, though 

 important, had left the subject in an entirely unsatisfying con- 

 dition, and then proceeds straight to the subject in hand. 



: c j << The minute details, both of the general solar surface and 

 of the extraordinarily complex spots, are one by one satisfac- 

 torily and lucidly described, -vith indications of the physical 

 conditions to which they are due ; and, finally, the general 

 bearings of all this on the received solar theories are briefly 

 set forth. We may fairly say that this paper is fundamental. 

 It treated of a subject of which little had been actually 

 known, and it leaves this subject in a satisfactory and settled 

 condition." 



His detailed study of the distribution of the heat of the 

 solar surface was begun in 1870, with the thermopile. It re- 

 sulted in the discovery of the previously unknown thermo- 



